Implement FTP support over dmtcp: the dmftp library and the ftpd service

[JohnAmadis]

This repo now builds two modules. dmftp is a Library holding the protocol itself - an RFC 959 server and client sharing one control-line codec, one jail-safe path resolver and one transfer engine. ftpd is an Application that turns a unit file's args and an INI file into a running server. Only a Library can be enabled as another module's dependency, which is why the protocol does not live inside the service.

Both PASV and PORT data connections work in both roles, TYPE A really translates line endings, and the server answers the full command set a real client needs (LIST/NLST, RETR/STOR/APPE, DELE/MKD/RMD, RNFR/RNTO, REST, ABOR, SIZE, FEAT, STAT). Files go through the dmod SAL rather than dmvfs - the same calls dmell's ls/cat/cp use, and the only option available, since dmvfs publishes no dmf-get package.

Nothing here owns a thread. Outbound file data is pushed from dmtcp's on_writable callback, so a multi-megabyte RETR advances at exactly the rate the peer acknowledges data, with no timer and no polling loop.

Two loader constraints shaped the code, both documented in app/dmftp/docs/dmftp.md: every dmtcp callback is registered from dmftp_net.c (the rule dmtcp's own docs record), and the command dispatch table holds no pointers at all - a pointer inside a static initialized aggregate is never relocated, which is why the verb is packed into a uint32_t and dispatch is a switch on an enum id.

Tested: 34 steps for dmftp - the codec exhaustively, including malformed input a peer gets to choose, plus an end-to-end client session driven through the real dmnetbridge -> dmip -> dmtcp -> dmftp path - and 5 for ftpd's configuration contract.

Requires dmtcp with on_writable/dmtcp_send_space().